When you connect an external display to an iPad, the screen currently simply gets mirrored. That's fine, but it could be utilized in a much more useful way. A simple improvement would be to only show the currently edited photo on the external display, similarly to how LumaFusion does it. Even better, however, would be to also alter the iPad screen to only show the sliders and not the currently edited photo, since it's already visible on the external display. That way the sliders could be larger and/or you could fit more of them on the screen at the same time. The iPad screen would then be fully utilized as a kind of control panel while the external display would only show the edited photo so one could see it nice and large.
Apple documentation for supporting external displays on iOS apps
This feature has already been discussed here, but since it was posted as a discussion instead of an idea, I created this post so others can upvote it.